So far under "ShootableType.java", DamageVsLiving and DamageVsVehicles both take integers. They should take float values like ExplosionDamageVsLiving and ExplosionDamageVsDriveable under "GrenadeType.java". In addition, there does not seem to be a variable that multiplies explosion damage for guns that fire explosive munitions, only explosion radius. Either that or the variables used under "GrenadeType" are also applicable to explosive munitions and I just didn't know.

Demitto and co already made this change in their Flan's mod plus. So hopefully it'll get implemented in 1.8.